In recent years, interest in using microwave signals for applications in many industrial settings has grown dramatically. One such setting is the heating of paper or other planar materials. Slotted waveguides have long been used for exposing planar materials to microwave energy. It is well known in the art to use a slotted wave guide that has a serpentine propagation path in order to maximize the exposure area of sheets passed through the guide. See, e.g., U.S. Pat. No. 5,169,571; U.S. Pat. No. 4,446,348; and U.S. Pat. No. 3,765,425.
Currently, the use of serpentine slotted waveguides for heating planar materials has four particular drawbacks. First, the microwave signal attenuates as it moves away from its source. This attenuation versus propagation distance increases when lossy planar materials are introduced into the waveguide. As a result, a material fed into the waveguide through a slot is heated more at one end of a segment (closer to a source ) than at the other end (further from a source). Prior art structures have not made use of the slot's orientation as a means of addressing this problem. In a traditional serpentine waveguide, there is a field peak midway between two conducting surfaces. In the prior art, the slot is at this midway point. See, e.g., the disclosures of U.S. Pat. No. 3,471,672, U.S. Pat. No. 3,765,425, and U.S. Pat. No. 5,169,571.
A second problem relates to the distribution of the microwave energy. Because the magnitude of the electric field in a microwave signal has peaks and valleys due to forward and reverse propagation in the waveguide, planar materials fed through a slotted waveguide tend to experience hot spots. U.S. Pat. No. 3,765,425 (hereinafter, "the '425 patent") addresses this problem through the use of two disconnected waveguides that are interspersed with each other. At least one waveguide is equipped with a phase shifter to ensure that the hot spots in one waveguide occur at locations different than in the other waveguide. The disadvantage to this approach (aside from the expense of a phase shifter) is that sections of separate waveguide must lay on top of one another in order for planar materials to experience alternating hot spots as they pass through the entire structure. Furthermore, each distinct variation in phase requires an additional serpentine waveguide and an additional microwave source.
Another attempt to smooth out the effect of "hot spots" is disclosed in U.S. Pat. No. 5,536,921 (hereinafter, "the '921 patent"). Like the '425 patent, the '921 patent also depends on separate and distinct sections of waveguide. But instead of using one or more phase shifters, the '921 patent offsets its separated sections of waveguide by exactly a 1/4 of a wavelength. The disadvantage of this approach is that it requires more than one phase-controlled path. The '921 patent requires even more paths than the '425 patent. According to the '921 disclosure, each waveguide section for exposing materials is a separate wave path. Each such section requires its own point for launching the wave and its own terminating point. Each launching point inevitably has losses due to signal reflection.
Most importantly, the approach disclosed in the '921 patent does not allow for easy adjustment to adapt to a variety of materials. It will be appreciated by those skilled in the art that the actual length of a 1/4 wavelength is dependent on the material introduced into the waveguide. Therefore, the '921 patent teaches a device that must be built for a specific material. If the constructed device was used for a material with a different .epsilon..sub.r, the 1/4 offset and its benefits would be reduced or completely eliminated. For example, if the structure disclosed in the '921 patent were used on a material whose .epsilon..sub.r was different by a factor of 4 from the .epsilon..sub.r of the material for which the structure was designed, then the material would be exposed to similarly placed (rather than offsetting) hot spots. It will be also appreciated by those skilled in the art that to further smooth out the effect of hot-spots, it may be advantageous to space hot spots by less than a 1/4 of a wavelength. In sum, the '921 patent discloses only a 1/4 of a wavelength offset and does not disclose a readily adjustable structure.
A third problem with traditional waveguides for electromagnetic exposure relates to the field gradient between top and bottom conducting surfaces. This gradient does not pose a problem if the planar material is of an insignificant thickness. However, if the planar material does have an appreciable thickness, this gradient can lead to nonuniform heating. One way to overcome this problem is disclosed in Applicants' co-pending application Ser. No. 08/815,061. This co-pending application, which is herein fully incorporated by reference, discloses the advantages of a dielectric slab-loaded structure that elongates the peak field region in a single mode cavity. However, slab-loaded structures have not yet been adapted for exposure of planar materials.
A fourth problem relates to leakage of microwaves through the slot of a slotted waveguide. Energy leakage and radiation is a general problem for any microwave structure. The problem of radiation through open access points is magnified when the material being passed through the structure has any electrical conductivity. Such conductive substances (e.g., any ionized moisture in paper that is passed through a chamber for drying) can, when passed through a microwave exposure structure, act as an antenna and carry microwaves outside the structure's cavity.
Currently in the art, two approaches are taken to address the problem of leakage through the slots of a slotted waveguide. One approach is to enclose the entire slotted waveguide in a reflective casing. See, e.g., the disclosure of U.S. Pat. No. 5,169,571. This approach has drawbacks. If the reflective casing does not itself have access points that remain open during the delivery of a microwave field, then the feed-through process must be fully automated and must exist inside the outer casing. On the other hand, if the reflective casing does have access points that remain open during the delivery of a microwave field--as does the structure disclosed in U.S. Pat. No. 5,169,571--then there is still a problem of leakage through those access points.
A second approach is the use of a reflective curtain draped over the slot. Although such a curtain may reduce leakage, it may also tend to obstruct smooth passage of any material that is fed through the slot. Any contact between such a curtain and any material tends to disrupt the surface tension of the material. Moreover, damaging arcing may occur between the curtain and the material. Furthermore, a reflective curtain does nothing to reduce the problem of an electrically conductive material's tendency to act as an antenna--alone or in combination with a waveguide's exterior conducting surface--and thus radiate energy through the slot.
Chokes that prevent the escape of electromagnetic energy from the cracks between two imperfectly contacting surfaces are well known in the art. Particularly well known are chokes designed for microwave oven doors and waveguide couplers. See, e.g., U.S. Reissue Pat. No. 32,664 (1988). What has not been fully explored in the art is the use of the choke flange concept to reduce leakage through arbitrarily shaped access points that remain open during delivery of a microwave field. Although choke flanges have typically been used to reduce leakages through two imperfectly contacting surfaces, the present invention and co-pending application Ser. No. 08/813,061, incorporated herein by reference, each show that the choke flange concept can also be applied to leakage through arbitrarily shaped openings in a feed-through type structure.